1. Field of the Invention
This invention relates to a hydraulic-actuated brake system, especially to a method and an apparatus for damping tactile vibration caused by changes in hydraulic fluid pressure induced in the hydraulic fluid by an operational characteristic of a brake when the brake is being applied to brake a rotating object, such as a wheel of a motor vehicle.
2. Background Information
Hydraulic-actuated brake systems are used extensively as service brakes of motor vehicles. A representative brake system comprises friction brakes at individual wheels. Application of the brakes occurs when the plunger of a brake master cylinder is depressed by a driver of the vehicle pushing on a brake foot pedal. Hydraulic fluid is displaced from the master cylinder to the individual brakes at the individual wheels. The magnitude of hydraulic fluid pressure correlates with the degree to which the brakes are being applied, possibly being amplified by a power brake booster. Generally speaking, the more forcefully the brakes are being applied, the greater the pressure in the hydraulic fluid.
An example of a friction brake mechanism in a motor vehicle is a disc brake where friction pads are pressed against opposite faces of a brake rotor when the brake mechanism is operated by depression of the master cylinder plunger. Because the rotor is turning with the vehicle wheel, the forces applied by the pads to the rotor are effective to brake the rotating wheel, and hence slow the vehicle when the vehicle is moving.
Certain friction brake mechanisms may possess certain operational characteristics that induce pressure changes, or fluctuations, in the hydraulic fluid as they are being operated to apply the brakes. Certain brake-induced pressure changes cause tactile vibration, sometimes referred to as brake roughness or shudder, which may propagate within the vehicle to be sensed by a driver of the vehicle, who may deem the vibration objectionable, even when it has no bearing on the functionality of the brake system or any other vehicle system. For example, vibration may be transmitted to the steering wheel where the driver may feel it through his or her grasp of the steering wheel. Such tactile vibration may manifest itself as torsional vibration. To promote driver satisfaction with a vehicle, it is therefore desirable that a brake system be free of such potential sources of objection, and it is toward that end that the present invention is directed. The invention may also serve to avoid needless warranty and/or repair expenses.
A cause of brake-induced vibration may arise from the nature of the brake mechanism itself. In a motor vehicle that has disc brakes, a cause may be a small divergence of opposite faces of a disc brake rotor from parallelism, even though the respective surfaces of the faces may be perfectly flat. Such divergence creates small differences in the distance between the opposite rotor faces, i.e. rotor thickness, as measured at various locations around the circumference of the rotor. Such differences may replicate a function that is a sinusoidal function of their locations around 360.degree. of the rotor circumference.
When opposing pads of a disc brake are pressed against opposite rotor faces, the small non-parallelism of those faces may induce vibration in the pads that is in turn reflected as pressure fluctuations in the hydraulic fluid. Such fluctuations may be more noticeable when the brakes are being lightly applied, such as to gently slow a moving vehicle, in contrast to more forceful applications, like hard braking for stopping the vehicle. And as observed above, vibration may propagate within the vehicle and be felt by the driver.
As a practical matter, it may not be cost-effective to impose stricter manufacturing tolerances on a disc brake rotor for the purpose of reducing the possible degree of non-parallelism between opposite faces, and consequently it is believed that the present invention may provide a better solution.
A preliminary novelty search developed the following U.S. Pat. Nos.: 3,430,660; 3,757,825; 4,166,655; 5,165,864; 5,410,945; and 5,820,227. Certain of those patents are concerned with avoiding wheel lock-up during hard braking, and none is seen to address the phenomenon with which the present invention is concerned, much less provide a solution for attenuating possibly objectionable brake-induced tactile vibration to levels that are likely not to be deemed objectionable.